Core casting method



Dec. 15, 1964 Filed Jan. 3. 1961 E. M. LEACH 3,160,931

CORE CASTING METHOD 3 Sheets-Sheet 1 ALCOHOL ETHYL SILICATE HCL ACID WATE R MIX LIQUID BINDER REFRACTORY SOLIDS MIX AGE AT ROOM TEMPERATURE AND CHILL TO 10 F.

MIX LIQUID WITH REFRACTORY SOLIDS MIX BINDER STORAGE AT -|oE To 20E DIE FORMING INTO CORE SHAPES AT WARM DIE TEMPERATURES CORE SHAPES FIRED TO HARDNESS CORES USED IN INVESTMENT CASTINGS INVENTOR.

Y EDWARD A4. Lf/ICH B IM m y.

Dec. 15, 1964 E. M. LEACH CORE CASTING METHOD 3 Sheets-Sheet 2 Filed Jan. 3. 1961 INVENTUR. 50144420 M LACH ATTORNEY EFFECT OF TEMPERATURE ON GELATION TIME E. M. LEACH CORE CASTING METHOD Dec. 15, 1964 Filed Jan. 5. 1961 SHLHNIW BWIJ. NOLLVIBQ 5 Sheets-Sheet 3 M pr. mm:

United States Patent 3,166,931 :CQ RE CASTING METHQD Edward M. -Leach, Bunker Hill, End, assignor to Union Carbide Corporation, a corporation of New York Filed Jan. 3, 1961, Ser. No. 80,292 oClainrs. ((3.22-194) This invention relates to an improvement in metal casting methods and in particular to providing preformed cores for forming internal passages and hollow areas in metal castings.

In the precision casting of metallic articles in ceramic molds, internal passages in the castings are formed "through the use of ceramic cores placed in the mold, or

where cores are not suitable, by drilling and machining the finished casting to form the passages. The use of cores to form such internal passages in castings is preferred over drilling or other metal workingoperations because of the greater expense of the latter methods. I-Iowever, because of the small size and intricate configuration of many proposed internal passages in castings, workable cores can not be produced of the needed size and shape with the 'result'that the passages must be drilled out or the design of the casting changed ,to fit present day core-making capabilities.

In one precision casting process, the shell mold casting process, a pattern of the object to be cast is made in wax or plastic. Any cores needed to form internal passages in the article are'embedded in the Wax pattern in their proper location. The Wax pattern and core assembly is then coated with a shell by dipping the assembly repeatedly into slurries of refractory binders followed by a stuccoing with refractory particles. The cores are designed to protrude from the wax pattern inplaces so that these ends of the cores are anchored into the shell. A shell of sufiicient thickness, usually from A to inch, is built up around the Wax pattern, and the mold is then fired to hardness-the wax material melting or burning out, leaving a cavity in the shell of the desired configuration complete with cores to form internal passages in the casting. It is easy to see that the cores are the weakest part of this shell mold for the strength of the shell mold itself is easily built up by increasing the thickness of the mold while the cores are of unchangeable dimensions being of the thickness and Width of the proposed internal passageway. For this reason the design problems in regard to the shell mold compositions and processes are quite different from those problems involved in designing suitable core ccmpositionsand forming processes.

The compositions used to make the cores must be such as to give a high strength to the core, even in thin sections, and be capable of producing smooth finishes on the cores.

Besides the need for a core material having the necessary strength and surface forming qualities, the process of fabricating the cores must be such as will easily fit'into production line operations. There are several core forming processes of which two are outlined below.

Where a large number of castings are to be made, a metal die of the proposed core shape needed is produced. Then a substantially dry ceramic core material is pressure cast in this die. The die injection must be done under pressure to eliminate voids and to compress the material to a proper strength with the result that the expensive 7 metal dies are worn down by the abrading ceramic maprocess can be used with plastic, rubber, or ceramic dies.

Because of the low strength of such dies, pressure casting 3,lhh,93l Patented Dec. 15, 1964 methods cannot be employed and, instead, the ceramic core material is prepared in the form of a free flowmg slurry and is poured into the dies. After the core material hardenswithin the die, it-is removed. Thehardening time may varyfrom about 35 to 70 minutes, depending on the composition of the slurry material. This constitutes a series drawback to production economy, be-

cause only about 10 cores can be produced from each die per 8 hour day. The mere provision of a quick-setting material afi-ords no improvement in the process because such quick-setting materials must'be used when made, having little .or no shelf-life-another property required of materials used in production line operations.

It is the primary object of this invention, .therefore, to

provide a ceramic composition suitable for formingintricate cores of high strength and smooth surface finish. It is another object of this invention to provide aprocess for forming cores of the type described which process is blade-like core-s forming the internal passages;

"FIG. 3 is a perspective View of a small turbine blade containing an intricate core assembly; I

FIG. 4 is a perspective view of the core assembly shown in the blade of FIG. 3;

FIG. 5 is a chartshowing the relationship of gelation or hardening time of the core material and the die temperature in the forming of the cores by the process of this invention.

In accordance with the objects a process 'for forming ceramic castings is provided comprisingpreparing a liquid binder solution consisting essentially of alcohol, ethyl silicate solution, hydrochloric acid and Water, mixing with said liquid binder solution a refractory particle and binder solids mix consisting essentially of inert refractory particles, powdered feldspar, powdered lead borate glass frit, and magnesia, casting the resultingslurry at a slurry temperature no higher than room temperature in a die having a temperature from about F. to about -F., removing the solidified casting from the die and firing it to'hardness at a temperature between '1700 F. and 2460 F. for from 1 to 4 hours.

The proper control of temperature promotes a fast gelation of the slurry. By casting this slurry composition in a die heated to a temperature from 90 'F. to 140 F. a much faster gelation of the slurry occurs than the 35 to 70 minute gelation time of some other slurry compositions in other processes. The "temperature of the slurry should be no higher than room temperature room temperature here meaning the temperature slurry would take on after preparation if allowed to stand,.u su ally room temperature. As will be seen from FIG.'5 of the drawing, the use of a chilled slurry, i-.e., at a temperature less than-room temperature, in a heated die gives a faster gelation time.

It is also necessary to chill the slurry'where it is not to be used immediately after preparation. The mixing of the constituents of the liquid solution causes an exothermic reaction that raises the temperature 'of the liquid'solution to about-l30 F. The solids mix is not added to the liquid solution at this temperature or gela tion would start to take place uncontrollably in less than an hour. Therefore the liquid solution is preferably aged for about 12 hours and chilledto a temperature between 10," F. and 20 F. before the solids mix is added. The resulting slurry of liquid solution and solids mix is preferably itself chilled to a temperature between I F. and Runtil used. The slurry is generally cast at a temperature between 10 F. or 20 F. and room temperature, whichis generally no higher than More specifically, as isshown in FIG. 1, a process for forming ceramic cores for use in casting moldsis' provided comprising preparing at room temperature a liquid binder solution consisting essentially by weight of from about percent to 75 percent alcohol, from about 65 percent to 25 percent ethyl silicate solution, from about 4 percent to 12 percent water and from about 0.2 percent to 0.6 percent concentrated hydrochloric acid,

reducing the temperature of the'liquid binder'solution to atemperature between -10 Ffand 20 F. andrnixing therewith a refractory particle and binder solids-mix consisting essentially by weight of from 6 percent to 9- percent powdered feldspar, from about 1.5 percent to 3.5 percent powderedlead borate glass frit from about 0.05 percent to 4.5 percent magnesia and the balance substantially all particulated refractory materials se l lected from the group consisting of silica, alumina, zircon, and firebrick grog, the amount of solids mix and,

liquid solution being about 70' percent to 85 percent by weight solids mix and the balance liquid solution, casting the resulting slurry at a slurry temperature no higher 7 surface and dimensional quality, and. inertn'ess to the metals and alloys being cast. v

A preferred liquid solution contains by weight from 48 to 54 perccnt alcohol, from 39 to percent ethyl silicate solution, from 5.5 to 7.5 percent water, and from 0.3 to 0.5 percent" concentrated hydrochloric acid. A typical liquid solution contains 51 percent by weight i507 propyl alcohol, 42 percent by Weight ethyl'silicate grade 40, 6.6 percent by weightwater, and 0.4 percent by weight concentrated hydrochloric acid. The specified proportions of alcohol and ethyl silicate'provide the lowtemperature' binders while the water and acid coact as the gelation time control agents and the hydrolysis catalyst, respectively. Ethyl silicate grade 40 is generally used, containing about 40 percent by weight SiO Other grades of ethyl silicate maybe used provided theycontainjat least 28 percent by weight SiO Common commercial denatured alcohol and'in particular isopropol alcohol is recommended. However other similar solvents may be used. Hydrochloric acid is preferred but other mineral acids maybe used. j

While the ethyl silicate contentprovides a low'temperature binder-for green strength, the solids mix con-r tains feldspar and lead borate glass frityfor high tern pcrature bonding. I

The high temperature binder feldspar, which is added as a powder, is present in amounts from 6 to 9 percent by weight.{ The high temperature binder lead bora'te percent by weight magnesia ofaparticlesize less than 200 mesh-is added to the solids-mix toneutralize acids and establisha slurry pH at least about 7, thereby promoting gelation of the slurry. a

The balance of the solids mix contains refractory par tides in specified particle sizes and of the following materialszjzircon, silica, alumina, and firebriclg grog. Of

special importance in producing the smooth-surfaced yet strong core castings of this invention is the inert nature of the selected refractory material and the range of refractory particle sizes. The refractory materials used, zircon silica, alumina, and firebrick grog are all chemically inert manufactured fused products which have no inherent bonding characteristics. It is important that these materials be chemically inert and without inherent I rials arebased on a blend of at least two different ranges of average particle size. The use of these blends guarantee a casting which is substantially lower in voids than when only one size range of particle is used in the slurry. The absence of large numbers of voids accounts for a smoother core surface because of the absence of surface depressions caused by voids or spaces between larg particles.

It is to be noted that in the manufacture of molds using slurries and refractory particles, it is common to produce smooth surfaces by decreasing the particle size of the refractory materials. In the case of shell molds the particle size of those layers which will be adjacent the metal are maintained small to provide a smooth surface while the outer layers are formed with larger particles to give strength. While such a procedure is acceptable with a shell mold or investment mold because the mold thick- I ness can boot unlimited size, it would not necessarily work with a small core casting which must have strength as well as smoothness in a limited and uniform cross-section. Thereforea blend of small and large particle sizes is generally used. The broad and narrow ranges for these particle size distributions are given below in Table I for each of the refractory materials. Also given is a typical solidsmix composition, it being understood that the exact compositions are selected from the broad ranges in accordance with such considerations as the configuration, size and weight of the individual core. The particle sizes referred to are of the standard Tyler Sieve Series.

glass frit, also added as a powder, is present in amounts 7 i from 1.5 to 3.5 percent by weight. .From 0.05 to 4.5'

TABLE ,I L

Solids Mlxes Typical Broad Preferred Compost tion I Zircon-Base Solids Mix: 1 v

Zircon Flour ('-200 Mcsh) 30 to 70 45 to 55 50. 5 Zircon Sand 00 Mesh) 70 to 30 35 to 43 38. 25 Magnesia. (-200 Mosh L 0. 05150 4. 5 1 5 to 2 1. 25 Lead Borate Glass Frit V I (powder) t 1. 5 to 3. 5 1 5 to 3 2 5 Feldspar (powder) 0 to 9 6 to 8 7 5 Silica' Base Solids Mix: i

, Silica Floor (-68 l /Iesh);..; 22 to 6D 30 to 40 35 Silica Flour lVIesh) 50 to 20 3t to 44 39 Silica Flour (-325 ldesh 8 to 15 10 to 13 12. 25 Magnesia -200 Mesh) 0.05 to 4. 5 1. 5 to 2 1. 75 Lead Borate' Glass Frit V ,(powder) 7 1.5 to 3. 5 1. 5 to 3 2. 5 Feldspar (powder) 6 to 9 6 to 8 1 7. 5

. Alumina-Base Solids Mix:

,, Powdered Alumina (60 p Mesh) 42 to 68 50 to 60 55 Powdered Alumina -,(35 a I Mesh) i 22 to 12 14 to 20 r 17 Crushed Fircbrick Grog ('-28'|50 MIosh) 12 to 22 14 to 20 l7 Magnesia (-200 Mesh) V 0.05 to 4. 5 V 1. Sta 2 1. 7.3 Lead Borate Glass Frit (Powder) r 1.5 to 3.5 1.511113 Feldspar (Powder)..- .6 to 9 6 to 8 TABLE 'ICont1nued Typical Broad Preferred Compositron Firebrick Grog-Base Solids Mix:

Crushed Fircbrick Grog +48 Mesh) 10 to 30 to 2:: Crushed Firebrick Grog (-50 Mesh) 20 to 50. to 30 'XXX Silica Flour 325 Mesh) -20 to 40 to 50 45 Magnesia 0.05110 4:5 1. 5 to 2 1. Lead Borate Glass Frit (Powde 1. 5 to 3. 5 1.5 to 3 2. 5 Feldspar (Powder) 6 to 9 6 to 8 7. 5

The amount of solids mix added to the liquids mix to form the slurry is broadly from about 70 percent to percent by weight solids mix and the balance liquids mix. A preferred relationship is 77 percent by weight solids mix and the balance liquids mix for the zircon-base mix and the following percents by weight solids mix for the other refractory bases: silica-base, 72 percent; alumina-base, 80 percent; firebrick grog, 83 percent.

As an example of the practice of the invention a liquid binder mix was prepared by mixing at roomtemperatures "51 percent isopropol alcohol, 42 percent ethyl silicate grade 40, 6.6 percent water, and 0.4 percent concentrated hydrochloric acid. The combination of these ingredients effects an exothermic reaction that increase the temperaadded to the chilled liquids and stirred for about 5 to 10 minutes and then the slurry is chilled to a temperature from -10 F. to +20 F. until used. The solids mix was zircon-based and consisted of 5 0.5 percent zircon flour (200 mesh), 38.25 percent zircon sand (80 +200 mesh), 1.25 percent magnesia (-200 mesh), 2.5 percent lead borate glass frit (powder), and 7.5 percent powdered feldspar. About 77 percent by weight solids mix Was added to 23 percent liquids mix.

Dies were prepared of the core shapes to be cast which Since U were similar to those shown in FIGS. 2, 3 and 4. the casting method didnot involve pressure casting, the dies could be made of rubber, plastic, ceramic or glass. The dies here were made of an epoxy resin. The slurry had a temperature of about 10 F. at the time of casting and the die had a temperature of about 140 F. Gelation occurred almost immediately. The core was removed from the mold and fired at2000 F, for about 2 hours. This firing cycle of 2 hours at 2000 F. is thepreferred cycle.

It is to be noted that the slurry need not be used immediately after preparation, but may be stored for a considerable period of time at a temperature between 10 and 20 F. Ordinarily the slurry will gel and harden at room temperature in about one hour; however, at about 10 F. the slurry remains fluid and usable for at least 240 hours. Because of this property, large batches of slurry may be prepared in advance'and stored until needed, instead of the normal practice of preparing a smaller batch every hour. 'This feature contributes to improved continuous mass production operations at a lower cost and insures uniformity .of product.

The gelation time of the slurry is a function of the heat transfer characteristics of the die-slurry system. In a series of tests to determine the effect of temperatureon thegelation time, the temperatures of thexdie andslurry were varied and all other known variables-were keptconstant, i.e., mass, volume, configuration and composition. By fixing these variables-the specificnheat, thermal conductivity and coetlicientyof heat transferof-the die and slurry are also heldconstant. The heat-transfer: fromdie to slurry is a direct function of the temperature difference. FIG. 5 shows the results of these tests. Note that the .gelation time is in direct relationship with the temperature difference of the die-slurry system. Because of this relationship, the gelation time may be accurately controlled, thereby. eliminating such variables as IOOIII'JZEIHPEIRTUIQ humidity and so on. These ambient variables have a pronounced eifect ongelationtime of currently used ma- .terials andmake continuous processing very -difiicultin the present art.

It is seen from FIG. 5 that.thedecreaseingelation time from the normal time of $35 .to 70-rninutestakesplace when'the die is at about F. If theslurryisat.10..F. the normal storage temperature, thenthe slurry .will.,gel in the 90 F. diein about 33-minutes. [The gelationtime shortens rapidly as the temperature difierence is increased by increasing .die temperature to 130 orl40-F. or: increasing the slu'rrytemperature slightly. Theminimum die temperature forra"10' F. slurry would be90 F. therefore. Generally the slurry will .be warmenthan 10 as a result of standingawaiting use. If therslurryisat 80 F. and the die at 'F., the. gelation -time will.be 18 minutes. As thedie. temperatureis increased to about 130 or 140 -F., the gelation time .decreasesrapidly. [In general the die temperature for s'lurries having aiemperature between 10 Fuand 80 F. willpreierablybe between about 100 Frand 140 F. .Thegelatioh timewill be shortest when the temperature diiference is'the. .greatest, i.'e., when the'die temperature is 130 F.or140-.F. -A specific die temperature can be selected which .suits'the needs of the specific production line involved. .Q'Ihis depends on the temperature of the slurrya'nd speed required inproducing castings. By using a die temperature between F.-and 140'F. almost immediategelation. can

be eifected.

The casting of the slurry compositions in the-heated die causes a co1ing-etfect Whichimproves surface quality of the cores. When the slurry is poured-"into' the heated die, 'the sudden-temperature change eifects'a-rapi'd gelation of the slurry that is immedately adjacenttothe inner wall of the die. If present, any evolutionfor 'accumulation of :gas bubbles will converge -:toward the center *of the slurry mass and will not form on the surface of the finished core where they "would cause irregularities.

The storage of the slurry for comparativelydong periods -of time does not effect the 'gelation or hardening time of the slurry except within the small ranges shown in Table II. A freshly made slurry will et inab'out the maximum time as shown in the ranges, while an aged slurry will .gel in about the minimum time. Aging-at 10 F., then is seen not 'to greatly aiiect therproperties of the slurry.

'TABLE II Die Slurry 'Temp. Gelation,v Condition Temp, Temp, Difier- Time, F. F. once :rnin.

80 10 70 3 5to 50 J l0 12 Bio '5' '80 80 -0: 25 to 30 130, 50

' Conditions A and B as shown in'Table II represent slurries that were cooled to 10 F. before casting. Cona formed by drilling of the castirig rather By providing the above described process for casting this class of materials, applicant has made it possible I .to accurately predict and control a convenient gelat-ion rate that will promote rapid and efficient production as well as goodquality control and lower production costs.

The-slurries produced according to the specifications above can be cast into core shapes of the most intricate configurations. The core shapes have sufficient green strength after casting to maintain themselves during further handling. After firing, the cores are strong enough to withstand rough handling and can be used in pres.- sure casting methods. The cores are compatible with the liquid metal, have a lowv anduniformly predictable rate of thermal expansion, and are readily removed from the casting by chemical means, sand blasting, etc.

In FIGURES 2, 3 and 4 two cores are shown which a were produced by the process of this invention. FIG. 2 shows a wax pattern and ceramic core assembly 11 for use in making a turbine stator blade. which were produced according tothis invention are embedded in the wax pattern 13 of the blade to form three 12, extending out of the wax pattern, are now embedded in. the shell mold so that on removal of the wax pattern 13 by melting or dissolving, a cavity is left in the mold I with the three ceramic cores'properly positioned therein. After the mold is fired, molten metal maybe poured in forming a blade of the same configuration as the-original wax pattern except that the cores 12 are removed leaving the internal passages in their place. Theblade shown 'in FIG. 2 is about 6 and /2 inches high and has a curved airfoil-shaped cross section. The cores are each'about 6 inches long, 1 and /Z inches wide at the tip and have a thickness varying from to Vs inch. These cores all have the same curves and twist as the airfoil shaped blade. Previously blades of this size and'shape could, not be made by the .lost wax process'because cores of such a length. and thinness couldnot be made-str'ongf and smooth enough. ,To make such a blade, sheet stock was formed around. spaced ribs (the. coolant passages being the spaces between the'ribs), and welded together.

With the compositions and processes of this invention,

cores are now producible thereby making possible the more rapid production of this type blade.

In.FIGURES 3 and 4 anothergform of core "is shown which also could not be produced successfully until the advent of this invention. FIG. 4 shows a wax pattern and core assembly 14 of asmall turbine blade-about 1 Three cores 112,

$5 whole structure coated with ceramic slurries and particles to form a mold. When the wax pattern is melted out the core assemlby remains firmly anchored in the walls of the mold. Prior art attempts to cast such coolant passages with composite "quartz-ceramic cores were unsuccessful because of the low strength of the cores and the failure of assembly at the ceramic-quartz joint. It

was necessary to cast a solid blade and then to drill out the internal passages.

It is to be noted that while the compositions and proc esses of this invention have been described in terms of the shell molding art and investmentcasting art, that the core forming compositions and processes of this invention can be used in other precision casting processes as well as in said casting and other founding methods.

What is claimed is:

l. A. process for forming'ceramic cores for use in casting molds comprising preparing a liquid binder solution consisting essentially by weight of from about'35 percent to 75 percent alcohol, from about 65 percent to percent ethyl silicate solution, from about 4 percent to 12, percent water and from about 0.2 percent to 0.6 percent concentrated hydrochloric acid, reducing the temperature of the liquid binder solution to a temperature between l0 F. and 20 F. andmixing therewith a refractory particle and binder solids mix consisting essentially by weight of from 6 percent to 9 percent by weight powdered feldspar,'from about 1.5 percent to 3.5 percent powdered lead borate I mix and liquid solutionbeing from about percent to percent by weight. solids mix and the balance liquid solution, maintaining the resulting slurry at not greater than room temperature until used, casting'the resulting -slurry at a slurry temperature no higher than room temperature in a .die having a temperature from about F.

to about 140 F., removing the solidified core from the die and firing it to hardness at a temperature between about 1700 F. and 2400 F. for from 1 to 4 hours;-

:- percent concentrated hydrochloric acid, reducing the temperature of the liquid binder solution to about l0 F. and mixing therewith a refractory particle and binder solids mix consisting essentially by weight of from 6 percent to 9 percent by weight powdered feldspar, from about 1.5 percent to 3.5 percent powdered lead borate glass frit, from about 0.05 percent to 4.5 percent magnesia and the balance substantially all particulated refractory materials selected from the group consisting of zircon; silica,

' alumina,.and firebrick grog, the amount of solids mix and inch wide and about 2 inches high. The core assembly 15 of FIG. 3 is shown embedded in the wax-pattern 16;. This fork-shaped core assembly represents the intricate arrangement of coolant passages in the blade (shown as dotted lines in the wax pattern of FIG. 4.).

The small vertical holes in the airfoil section of the' blade have a diameter of only. 0.030 inch and are usually of cores. V

The core assembly 15 is madeup of a T-shaped secthan by the use a liquid solution being about 70 percent to 85 percent by weight solids mix and the balanceliquid'jsolution,niainand tiring it to hardness-atatemperature.between about 1700 F. and2400? F. for from l to 4 hours.

.3. "The process of claim 2 whereinjsaid selected refractory material is zircon and the amount of solids mix and liquid. solution being about 7 7. percentv by, Weight solids mix and the balance liquidsolutiom.

4. The process of claim 2 wherein said selected refracltory material is silicaand the amount of solids mix and "liquid solution beingabout 7 2' percent by weight :s'olids inix and the balance liquid solution. I

5. The process of claim 2 wherein said selected refrac- References Cited in the file of this patent tory material is alumina and the amount of solids mix UNlTED STATES PATENTS and liquid solution being about 80 percent by weight a solids mix and the balance liquid solution. 2521839 reag-m Sept' 1950 2,732,600 Hanlnk et al Jan. 31, 1956 6. The process of claim 2 wherein said selected refrac- 5 2,818 619 Bradley et a1 Jan. 7 1958 tory material is firebrick grog and the amount of solids 2 92 2 Opel-hall June 1959 mix and liquid solution being about 83 percent by weight 2,911,310 Shaw et a1 Nov. 3, 1959 solids mix and the balance liquid solution. 3,032,425 Leach May 1, 1962 

1. A PROCESS FOR FORMING CERAMIC CORES FOR USE IN CASTING MOLDS COMPRISING PREPARING A LIQUID BINDER SOLUTION CONSISTING ESSENTIALLY BY WEIGHT OF FROM ABOUT 35 PERCENT TO 75 PERCENT ALCOHOL, FROM ABOUT 65 PERCENT TO 25 PERCENT ETHYL SILICATE SOLUTION, FROM ABOUT 4 PERCENT TO 12 PERCENT WATER AND FROM ABOUT 0.2 PERCENT TO 0.6 PERCENT CONCENTRATED HYDROCHLORIC ACID, REDUCING THE TEMPERATURE OF THE LIQUID BINDER SOLUTION TO A TEMPERATURE BETWEEN -10*F. AND 20*F. AND MIXING THEREWITH A REFRACTORY PARTICLE AND BINDER SOLIDS MIX CONSISTING ESSENTIALLY BY WEIGHT OF FROM 6 PERCENT TO 9 PERCENT BY WEIGHT POWDERED FELDSPAR, FROM ABOUT 1.5 PERCENT TO 3.5 PERCENT POWDERED LEAD BORATE GLASS FRIT, FROM ABOUT 0.05 PERCENT TO 4.5 PERCENT MAGNESIA AND THE BALANCE SUBSTANTIALLY ALL PARTICULATED REFRACTORY MATERIALS SELECTED FROM THE GROUP CONSISTIN OF ZIRCON, SILICA, ALUMINA, AND FIREBRICK GROG, THE AMOUNT OF SOLIDS MIX AND LIQUID SOLUTION BEING FROM ABOUT 70 PERCENT TO 85 PERCENT BY WEIGHT SOLIDS MIX AND THE BALANCE LIQUID 